This invention relates to a method for accurate magnetization of tubular wellbore members such as casing segments or drill string segments. Such magnetization produces a remanent magnetic flux that extends at a distance or distances from the wellbore member, about that member, to facilitate detection of such a tubular member in a borehole when drilling another borehole, for example in an attempt to intercept the borehole containing the magnetized wellbore member.
The prior art discloses methods to determine the location and attitude of a source of magnetic interference such as a magnetized wellbore tubular having a remanent magnetic field. In this regard, U.S. Pat. No. 3,725,777 which describes a method to determine the earth""s field from a magnetic compass and total field measurements, and then calculate the deviations, due to the external source of magnetic interference. The magnetic field of a long cylinder is then fitted to the magnetic deviations in a least-squares sense. That ""777 patent, and the paper xe2x80x9cMagnetostatic Methods for Estimating Distance and Direction from a Relief Well to a Cased Wellborexe2x80x9d, describe the source of the remanent magnetic field. The ""377 patent states, at column 1, lines 33 to 41 that xe2x80x9cTo have a remanent magnetization in the casing is not difficult since most well casing is electromagnetically inspected before it is installed. The electromagnetic inspection leaves a remanent magnetization in the casing. Since casing is normally installed in individual sections that are joined together, the remanent magnetization of unperturbed casing is normally periodic.xe2x80x9d
U.S. Pat. No. 4,072,200 and related U.S. Pat. No. 5,230,387 disclosed a method whereby the magnetic field gradient is measured along a wellbore for the purpose of locating a nearby magnetic object. The gradient is calculated by measuring the difference in magnetic field between two closely spaced measurements; and because the earth field is constant over a short distance, the effect of the earth field is removed from the gradient measurement. The location and attitude of the source external to the drill string can then be determined by comparison with theoretical models of the magnetic field gradient produced by the external source.
U.S. Pat. No. 4,458,767 describes a method by which the position of a nearby well is determined from the magnetic field produced by magnetized sections of casing. U.S. Pat. No. 4,465,140 describes a method for magnetization of well casing. In this method, a magnetic coil structure is traversed through the interior of the casing, which is already installed in the borehole. While traversing the casing, the coil is energized with a direct current which is periodically reversed to induce a desired pattern of magnetization.
European Pat. No. Application GB9409550 discloses a graphical method for locating the axis of a cylindrical magnetic source from borehole magnetic field measurements acquired at intervals along a straight wellbore.
U.S. Pat. No. 5,512,830 describes a method whereby the position of a nearby magnetic well casing is determined by approximating the static magnetic field of the casing by a series of mathematical functions distributed sinusoidally along the casing. In an earlier paper xe2x80x9cImproved Detectability of Blowing Wellsxe2x80x9d, John I. DeLange and Toby J. Darling, xe2x80x9cSPE Drilling Engineeringxe2x80x9d, Society of Petroleum Engineers, Mar. 1990, pp. 34-38, a method was described whereby the static magnetic field of a casing was approximated by an exponential function.
European Patent Specification 0 031 671 B1 describes a specific method for magnetizing wellbore tubulars by traversing the tubular section in an axial direction through the central opening of an electric coil prior to the installation of the tubular section into a wellbore. Production of opposed magnetic poles having a pole strength of more than 3000 microweber is disclosed.
The above referenced paper xe2x80x9cImproved Detectability of Blowing Wellsxe2x80x9d, expresses the need for as high a magnetization as possible in the target tubulars, and states, xe2x80x9cBecause most magnetometers in use in survey/MWD have a sensitivity of +/xe2x88x920.2 microTesla, a value of 0.4 microTesla is considered to be a reasonable threshold value.xe2x80x9d Note that 0.2 microTesla is equivalent to 200 nanoTesla, and that in the patent and the paper, a lower limit to the tubular magnetization, namely 3000 microweber, is described or claimed.
It is one objective of the present invention to take advantage of improvements in the state of the art of magnetometer measurements to provide a method of magnetization of wellbore tubulars for use in drilling intercept wells that does not require such a high level of magnetization as 3000 microWeber.
The value of 0.4 microTesla cited in the above referenced paper for good detectability of small magnetic field changes was representative of the state of the art in magnetometer measurements at the time of publication of that paper in 1990. The present invention employs a magnetometer sensor and electronics apparatus for borehole use having a 16-bit analog-to-digital converter enabling much higher accuracy and resolution characteristics. This leads to a quantization of about 2 nT (nanoTesla) per bit that in turn leads to a root-mean-square quantization error of about 0.58 nt RMS. Other electrical noise in the system as well as basic magnetometer noise limits the detectability of small changes in magnetic field to about 2 nT with short-term averaging of the measurements. This value, 2 nT, is thus 200 times less than the 400 nT cited in the referenced paper as a xe2x80x9creasonable threshold.xe2x80x9d Thus, either the range of detection of a magnetic target can be greatly increased for a given magnetization of the target tubular, or the magnetization of the tubular can be substantially reduced from previous values required by prior art.
Reduced required magnetization of the tubular results in reduced size and weight for the magnetizing apparatus, reduced electrical power for the magnetizing apparatus, reduced sideways-directed forces between the magnetizing apparatus and the tubular during magnetizing and reduced magnetic forces between the individual tubular element and other magnetic materials during handling, prior to insertion into the borehole.
The reduced electrical power for the magnetizing apparatus makes it possible, in some embodiments, to measure the magnetic pole strength of the induced magnetization and if desired control the electrical power to achieve a controlled and known level of magnetization. Such a known level of pole strength of the magnetization can lead to improvements in the estimation of range to the target casing in the intercept process.
Accordingly, the method of the invention includes, in some desirable embodiments, either or both:
1. Measuring the induced pole strength of the induced magnetization in the tubular element;
2. Measuring the induced pole strength of the induced magnetization in the tubular and using such measured pole strength, in feedback relation with the electrical power of the magnetizing apparatus, to control the magnetization to a desired level, in the tubular element.
It has been well known since 1971, the filing date for U.S. Pat. No. 3,725,777, that a useful remanent magnetic field in wellbore tubulars can be obtained as a by-product of magnetic inspection of the tubular prior to installing the tubular in a borehole, such inspection involving applying a magnetic field to the tubular element. This invention expands on that knowledge by describing how specific requirements on magnetic field values during the inspection process can produce the desired levels of magnetic pole strength for the tubular, without requiring a separate specific apparatus or procedure following magnetic inspection.
Major objects of the invention include providing for well tubular member magnetization, by carrying out the following steps:
a) providing a magnetizing structure comprising an electrical coil defining an axis,
b) relatively displacing said tubular member and said structure, with said coil positioned and guided in close, proximity to said member, and while supplying electric current to flow in the coil, thereby creating magnetic flux passage through said tubular member and core to magnetize that member, or a part of that member,
c) and displacing said tubular member in a wellbore.
In that method, the coil may remain positioned either externally or internally of the member during such relative displacing of the member and structure. Further, a spacer element or elements, as for example a roller or rollers, may be provided for spacing the coil from the tubular member during such relative displacing of the member and structure.
Additional objects including providing flux passing pole pieces at opposite ends of the coil; measuring the magnetic pole strength of the magnetic field produced proximate the end or ends of said member, by said flux passage; and controlling a parameter of the flux as a function of such measuring; and magnetizing the tubular member to a pole strength less than about 2,500 microWeber.
Further, the method includes and facilitates magnetically detecting the presence of the member in the wellbore, from a location outside the bore and spaced therefrom by underground formation. Also, the method may include providing a magnetic measurement device, and displacing that device within said member in the wellbore while operating the device to enhance magnetization of the member, in the well.
The tubular member may comprise any of the following:
i) a well casing section
ii) well tubing
iii) drill pipe.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: